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UNITED STATES DEPARTMENT OF AGRICULTURE 




f BULLETIN No. 1081 




Ji\mr^ "^M^U 



Washington, D. C. 



PROFESSIONAL PAPER 



JUL V, 1922 



BIOLOGY OF THE PAPAYA FRUIT FLY, TOXOTRY- 
PANA CURVICAUDA, IN FLORIDA. 

By Akthur CViNlAsox, Assistant Entoinologist, Fruit-lnscct luiestiyations, 
" Bureau of Entojiioloay. 



CONTENTS. 



Introduction 

Description and life history 

Seasonal history and occurrence 

Power of flight 



Page. 



Susceptibility of varieties 8 

Enemies 9 

Control measures 9 



INTRODUCTION. 

The occurrence in 1905 of a newly introduced species of fruit fly 
(Toxotrypayia curvicauda Gerst.) ^ attacking the papaya {Carica 
papaya L.) in south Florida was the occasion several years later for 
a preliminary investigation and paper on the insect by Knab and 
Yothers.- Technical descriptions of the insect, together with its dis- 
tribution and history, are recorded therein, and also some notes 
on its habits. In recent years the pest has assumed much greater 
economic proportions, due to the increasing production of the papaya 
from a commercial standpoint, and also to the spread of the insect 
over nearly all portions of the State where papayas are grown. 
Hence a careful study of its biology and control was undertaken by 
the writer. 

It is the purpose of this bulletin to present an accurate account 
of the life history .md seasonal historj^ of the insect, together with its 
habits and the factors influencing its development and spread. The 
methods of control as far as worked out are also given. 

' Dipterous family Trypetidae. 

= Knab, Frederick, and Yothers, W. W. Papaya fruit fly. In Jour. Agr. Research, 
vol. 2, No. 6, p. 447-454, pi. 41-42. 1914. Literature cited, p. 453. 
101920—22 1 



Q/^jlJJ, 



2 BULLETIN lOSl, U. S. DEPARTMENT OF AGRICULTURE. 

DESCRIPTION AND LIFE HISTORY. 

EGG. 

The eggs (PL I, C, h) are of very unusual proportions, being long 
and slender, somewhat club-shaped or fusiform, with a long cylin- 
drical stalk. The average length is about 2.5 mm. and the greatest 
diameter is 0.2 mm. They are inserted into the seed cavity of the 
fruit from the long ovipositor of the female. The stalk sometimes 
remains partly in the flesh, although the eggs are never placed 
there as the young maggots seem unable to survive there. They 
always occur in clusters, and usually there is only one cluster to a 
fruit. The cluster consists of from 6 to 20 or more eggs, which are 
always fastened together by an adhesive substance on the surface 
of the eggs. One female, according to Knab and Yothers, is capable 
of laying 103 eggs, all of which are disposed of at about Uie same 
time. 

The eggs require from 12 to 14 days to hatch at any time through- 
out the year. Although the otlier stages are longer in wdnter than 
summer, the o^gg seems not to be affected by climatic changes. This 
point was determined by cutting open infested fruits at definite 
intervals after they were stung. Usually an adult would oviposit in 
several fruits on a tree the same evening. It was then possible to 
cut one of these on each of several successive daj^s until the eggs 
were found to liave hatched. Even in the fruits which had been 
cut the eggs would complete their development if the halves were 
I^laced together, provided they were several days old when first 
exposed to the light and air. With freshly laid eggs this was not 
found to be true. Many attempts were made to rear the eggs arti- 
ficially after removing them from the fruit liut without success. 
When dissected from the fruit and placed on a piece of leaf or 
fruit puljD over a plug of wet cotton in a vial inverted in water, 
as practiced by Back and Pemberton^ with melon-fly eggs in Hawaii, 
they failed to develop. Even though the conditions of heat and 
moisture were apparently the same as in the fruit they did not 
hatch. 

When ready to hatch the eggs split longitudinally along the 
micropylar half and the maggot escapes, leaving the stalk end intact. 



The young maggots on hatching from the eggs begin at once to 
feed on the coating of the seeds. They remain for about the first 
half of their existence w^ithin the seed cavity, feeding on the seed 
coverings and other fibers there. Many of the seeds , become de- 

= Baek, E. A., and Pemberton, C. E. 'The melon fly in Hawaii. U. S. Dept. Agr. Bui. 
491, p. 18. 1917. 



._-..u--^/iia5J 






^T\ --- Kit/i BIOLOGY OF THE PAPAYA FRUIT FLY. 3 

tachecl by this process, and the loose seeds in the fruits serve as an 
indication of their presence. "When newly born the luagjTots are 
almost transparent, but soon assume a shinino-. dirty white color 
while in the seed cavity. Later on, as they continue to develop, they 
eat into the flesh of the fruit, first close to the cavity and then work- 
ing farther out until, when mature, they are close to the skin. They 
have then only to eat a hole through the rind to escape. During this 
latter part of their life they become a rich golden yellow color, like 
the color of the fruit on which they are feeding. The presence of 
the maggots in the fruit usually causes it to turn yellow and ripen 
prematurely. This is a distinct advantage to the larvse, for they 
do not like the juice of the green fruits and usually remain around 
the seed cavity until the flesh begins to soften. 

The mature maggots (PI. I, C, a) average about 11 mm. in length, 
are subcylindrical in shape, and taper anteriorly to the mouth. 

The length of time required for their development varied from 10 
to 27 days in a large number of tests. The cooler weather of winter 
})rolongs somewhat the length of the larval stage. Conditions un- 
favoral)le to the larva?, such as the fruit decaying or the maggots be- 
ing removed from the fruit, will cause them to transform before the 
normal time. On the other hand, if the conditions are favorable 
the larvpe may remain in the fruit for several days after reaching 
maturity. The average time for this stage is 15 dav'S. 

They make their escape by eating a hole through the skin and drop- 
ping to the ground. As a rule, when one escapes the others will 
follow in rapid succession, and often all emerge from the same exit 
hole. If the fruit has already fallen from the tree the maggots go 
into the groimd innnediately under it: if the fruit is still on the tree 
they drop to the ground. Often a larva will remain partly emerged 
from a fruit and continue a wriggling, twisting motion for an hour 
or more before finally dropping. When once on the ground the 
maggots immediately bury themselves and never wander around on 
the soil. The transformation is completed within a few hours after 
entering the ground. The period of exposure from the time of leav- 
ing the fruit to entering the soil ordinarily would be only a minute 
or two, and consequently there would be little chance for parasitism 
here, ^^erv rarely a maggot will pupate inside a fruit. 

The number of maggots in a single infested fruit sometimes runs 
up as high as 40, although ordinarily there are about 15 or 20. A 
very small fruit may have only 2 or 3. 

If confined in breeding jars where no soil is present the larvae 
usually will not pupate. In a glass stender dish or Petri dish the 
mature maggots would remain in the larval stage for three or four 
days, continually crawling around the dish. After several days they 



4 BULLETIN 1081, U. S, DEPARTMEaSTT OF AGRICULTURE. 

attempt to pupate, but many of them die before completing the trans- 
formation. Even when they succeeded in pupating, the adults never 
matured from them. Evidently this is due to a lack of moisture, 
which seems to be a vital factor to all stages of development in this 
insect. 

PUPARIUM. 

In common with other fruit flies, this insect passes the pupal stage 
in the ground. The puparia occur naturally under the infested 
trees in the soil, for, as stated above, the maggots do not travel 
around, but go into the ground where they fall. The average depth 
of the puparia is 2 inches, although they vary anywhere from the 
surface to 8 inches deep, and sometimes occur also under rock and 
rubbish on the surface. The moisture in the earth seems to deter- 
mine largely this point, for they go down until they can get into 
damjD soil. Very rarely one is found inside the fruit either on the 
tree or on the ground. 

The puparia (PL I, B) are of a stout, subcylindrical form with 
rounded ends and vary in length from 8.5 to 12 mm. The size is no in- 
dication of sex, for from 100 of the smallest ones obtainable about 
an equal number of males and females emerged. The color of the 
puparia varies all the way from a light ferruginous yellow to dark 
brown or almost black. This color in no way indicates their age, 
for some remain light colored throughout their existence. 

The pupal stage was found to vary from 18 to 44 days in breed- 
ing out several hundred in all months of the year. Aside from the 
temperature changes the effect of the moisture is a very large factor 
in this regard. Under favorable conditions of moisture the largest 
number of the adults will emerge after 18 to 20 days in hot weather, 
but in winter this runs up between 30 and 40 days on the average. 
Hooker ' found it to last from 17 to 21 days in Porto Rico. Mois- 
ture, even more than heat, seems to be the determining factor. Lack 
of moisture will prolong very materially the pupal stage and if con- 
tinued will prove fatal. On the other hand, excessive moisture will 
kill the puparia. The following data prove this point : 

One hundred fresh puparia phiced in soil in a jar and kept witliout any 
water beinjr added. All died. 

One hundred fresh puparia placed in soil in a jar and kept moderately moist ; 
80 adults emerged, 20 died. 

One hundred fresh puparia placed in soil in a jar and kept wet every day. 
All died. 

Even under the most favorable conditions of heat and moisture 
the puparia do not all mature, and 70 per cent is a very good aver- 
age. Under natural conditions the average runs below that. Sev- 

*, Hooker, C. W. Fruit flies, in Porto Rico Agr. Exp. Sta. Ann. Rpt. 1912, p. 36. 1913. 



Bui. 1081, U. S. Dept. of Agriculture. 



Plate I. 




6 







\J^ 



,„,^a3lfes=v.,,. 



TOXOTRYPANA CURVICAUDA. 

A, .\duU tlif.s: «, Ft'inuh'; /;, iiiiilc. ■ 2. li, I'lipuria. ■, \\. (', a, Larvii', ■, 2; h, egg.-^, ■, 20 



Bui. 1081 , U. S. Dept. ot Agriculture. 



Plate II. 





BIOLOGY OF THE PAPAYA FRUIT FLY. 5 

eral hundred puparia placed in jars, some of them reared from larvae 
and others gathered in the soil under trees, gave results as shown in 
Table I. 

Table I. — Males mid females of Toxotrypana curvlcauda maturing from puparia 

placed in jars. 



Number 
of pupae. 


Number 
of males. 


Number 

of 
females. 


Total 
emerged. 


Total 
died. 


870 


279 


281 


560 


310 



Thus it is seen that only 64.3 per cent matured into adults, while 
35.7 per cent died. Of the number maturing practically 50 per cent 
were males and 50 per cent females. 

Practically all the adults emerge from the soil in early morning 
just before dajdight. Very rarely will one emerge between sunrise 
and midnight. The adults often carry the pupal case to the surface 
of the ground before freeing themselves from it. Only a few minutes 
are then required for their complete development. 

ADULT.5 

The adult of this species (PL I, A) is a wasplike fly, very much 
resembling in coloration and general appearance the wasps of the 
genus Polistes. The body is yellow and brown, marked with black, 
and the females are made strikingly conspicuous by a long, curved 
ovipositor, even longer than the body itself. There is considerable 
variation in the size of the flies, but they average about 12 mm. in 
length. The ovipositor of the female varies from 10 to 14 mm. in 
length. 

The flies exhibit a rather rapid flight and Avalk with a quick 
nervous motion. The females are not often seen on bright days but 
appear about the trees to lay their eggs in the late afternoon or even- 
ing. They show a negative reaction to sunlight and always seek the 
shady side of the tree or fruit. Although sometimes seen during the 
morning and noon hours, the greatest flight occurs about an hour 
before sunset. The males, however, are more active on bright daj's. 
Both sexes are easily disturbed when resting on the fruits. 

The life of the adult flies is probably only a few days in length. 
They have been kept alive in captivity for 31 days when properly 
fed, although the average is very much less. The flies will eat any 
kind of sugar sirup and the pulp and juices of some fruits, but they 
never appear to be attracted by any food. Many will die without 

s For original descriptiou see Gerstaecker, A. Beschreibung einiger ausgezeicbneten 
neuen Dipteren aus der familie Muscariae. lu Eat. Zeitung Stettin, Jahrg. 21, No. 4/6, 
p. 194-195. 1860. 

101920— 22— —2 



6 BULLETIN 1081, U. S. DEPARTMENT OF AGRICULTURE. 

ever eating when food is placed at their disposal. Others will eat 
only when food is placed directly in front of them or when they hap- 
pen to walk into it. When they have once tasted the sweets they will 
feed until the body is well distended. The best results were obtained 
by placing drops of brown-sugar sirup on the net or screen covering 
of the cages. In the large cages it was sprayed with an atomizer on 
the under side of the leaves of the tree. The flies have a liking for . 
the pidp of ripe papayas and also eat bananas but will not eat the '• 
juice of oranges. 

The following figures show the length of life of some of the flies : 

Fifty-four flies confined without food after enierjiing- lived from 1.5 to 5.5 
days, witli an average of 3.45 days. 

Thirty-six flies given water only lived from 3 to 6 days with an average of 
4.6 days. 

Seventy-six flies fed on sugar sirup lived from 3 to 31 days with an average 
of 7.4 days. 

Under natural conditions these figures probably do not vary much. 
Five to seven days represent an average life for the adult. 

COPULATION. 

The insects copulate usually on the leaves or fruits of the papayas, 
but can only rarely be observed. Copulation takes place during the 
daytime, for the male is more active then, as noted above. He seems 
to experience some difficulty in holding the female in position because 
of the long ovipositor. To accomplish his object, he alights on top 
of the female and, clasping her body with the first two pairs of legs, 
he draws the ovipositor back and up with the remaining pair. Then 
by practically standing on his head he is able to bring the tip of his 
abdomen in conjunction with the end of the ovipositor. They usu- 
ally hold this position for several minutes or longer, one pair being 
observed to remain for nearly two hours. If disturbed the female 
will walk around the fruit or even fly to another tree, always carry- 
ing the male along in position. In captivity the flies very seldom 
copulate. This is true when confined in the large cages over the trees 
(PL II, at left) as Avell as in small cages or "jars in the laboratory-. O'f 
several hundred adults bred out in jars and observed at all hours of 
day and night, only a very few ever made any attempts at copulation. 
These cases happened when the flies were 4 or 5 days old and had been 
fed on sugar sirup or fruit pulp. If given no food they soon die 
without mating, 

OVIPOSITION. 

Oviposition usually takes place in the evening, that being the 
time the adidt females are most active. It has been observed 
occasionally, however, to take place at all other times of the day. 
The fruits selected by the females in which to lay their eggs are 



BIOLOGY OF THE PAPAYA FRUIT FLY, 7 

usually medium or larger sized, if all sizes are present on the plant. 
They often begin work on a plant when the fruits have just set and 
are very small, and all sizes of fruits are subject to attack. They 
seem, however, to prefer the half -grown or larger fruits, per- 
haps due to a natural instinct, for if the eggs are deposited in a 
nearly mature fruit, the fruit may ripen and decay before the mag- 
gots have completed their growth. On the other hand, if placed in 
very small fruits the maggots will mature before the fruit has 
started to ripen, and they sometimes experience difficulty in escaping 
from green fruits. It has been said that the milky juice from the 
green fruits is fatal to the larvae, but this has not been found to be 
true. In fact, maggots which had been rolled around in the juice 
from green fruits completed their development. 

It is not often that an adult fly will oviposit in a fruit where eggs 
or maggots are already present, although in a few instances maggots 
of two distinct sizes were found. When the first ones to mature 
escape they cause the fruit to decay, so the younger ones may not 
be «ble to complete their development. 
«MB* The adult fly alights on tlie fruit selected and usually walks around 
for a time with a nervous motion. When she has found a suitable 
place she forces her ovipositor througli the skin and flesh of the 
fruit and deposits her eggs inside in the seed cavity. This is ac- 
complished by raising the long ovipositor up in a curved position 
and placing the tij) of it on the fruit near the end of the abdomen, 
then forcing it through the fruit. The position taken is much the 
same as that of the ichneumon flies in depositing their eggs. 

The eggs are laid in clusters and ordinarily only one cluster will 
be placed in a single fruit, although occasionally two or three are 
found. The fly often stings a fruit several times, as many as 10 
punctures being counted at times, but does not always deposit a 
cluster of eggs. Possil)ly she is not able to reacli througli the flesh 
of the fruit in all places and hence withdraws and seeks a new place. 
In fact, many fruits are stung several times and no eggs laid in them. 
This has often happened in the breeding cages where fruits sup- 
posedly containing eggs failed to develop any maggots. Also many 
fruits on the trees have been marked after being stung and no 
larva> ever appeared in them. Usually about two minutes are re- 
quired by the female to deposit the eggs, although instances have 
been noted where the ovipositor remained inserted for an hour or 
more. Occasionally a female will become trapped and die in the 
milky juice which wells up when the skin is punctured. This exu- 
date coagulates and holds the fly if she does not soon escape. When- 
ever a fruit is stung the exudate produces a characteristic nuirk by 
runnino: down the side of the fruit and also coagulating in a large 



8 BULLETIN 1081, U. S. DEPARTMENT OF AGRICULTURE. 

drop at the puncture. (PI. II, at right.) It is possible thereby to de- 
termine easily the number and location of the punctures. 

SEASONAL HISTORY AND OCCURRENCE. 

The insects breed throughout the year in Florida and are present 
in all stages at any month of the year. They have, however, some 
seasonal preferences and occur in much larger numbers at some 
seasons than others. The time of greatest flight of the adults seems 
to be during March and April, while in late summer and fall there 
are very few of them in evidence. This is correlated largely with 
the growth of the host plants, which begin fruiting usually in the 
fall and continue through the winter and spring. Many of the 
plants die down or are cut out in the late spring and new ones set. 
The flies therefore appear on the new fruits in the fall and con- 
tinue to breed in increasing numbers throughout the winter and 
spring. The wild papaj^as in the hammocks fruit at all seasons and 
always serve as hosts whether or not any of the cultivated sorts 
are available. The generations are by no means marked and vary 
in length from 40 days in summer to 70 or more in colder weather. 
In a year's time there are about six generations, although they over- 
lap and are in no way distinct. Moisture in the soil is a very im- 
portant regulating factor in the length of all stages, perhaps even 
more so than changes of temperature. 

POWER OF FLIGHT. 

The distance which the adults are able to travel is not very great, 
for they are not strong fliers. One planting of papayas under obser- 
vation was placed 2 miles from where any other plants existed and 
remained free from infestation throughout the season, the adults 
apparently being unable to cover that distance. In most locations, 
however, there are wild papayas all through the surrounding ham- 
mocks, and these serve to harbor and spread them. 

SUSCEPTIBILITY OF VARIETIES. 

While no distinct varieties of the papaya {Carica pa'paya) are 
recognized, there are several types of the fruit grown in the State. 
Several have been introduced from foreign countries and crossed on 
existing types. Then there are the original wild plants which have 
been cross-pollinated on the cultivated plants through natural 
agencies. Through all this cross-pollination there result two rather 
distinct types of fruits, one the small, round, or oval type with 
rather thin skin and flesh and the other the large, oblong fruits which 
usually have thick flesh. One especially fine fruit of the latter type 
has been produced at the Plant Introduction Gardens at Miami, Fla., 



BIOLOGY OF THE PAPAYA FRUIT FLY. 9 

by Mr. Edward Simmonds, and is known as No. 28533. These oblong- 
fruits are much more immune to the attacks of the flies, due largely 
to the fact that the female flies are unable to reach through the flesh 
of the fruit with their ovipositors and lay their eggs. In fact, in some 
places they were found practically free from infestation and are 
considered immune by the growers. An examination of about 300 
fruits of all kinds on the Florida Keys, by A. L. Swanson, an in- 
spector of the State Plant Board of Florida, showed 90 per cent of 
infestation in the small round fruits as compared to no infestation 
in the large oblong fruits. This latter fact has not held good, how- 
ever, in investigations by the writer. Several hundred fruits ex- 
amined both on the keys and in many places on the mainland showed 
about 88 per cent of the round or oval fruits infested and about 15 
per cent of the oblong fruits infested. In wild fruits in the ham- 
mocks the infestation is close to 100 per cent. No papayas grown in 
the State are entirely immune from the attack of the flies. 

ENEMIES. 

Only two natural enemies have been noted on this insect, one the 
jumping spiders and the other the small red ants which sometimes 
prey upon the larv?e. The large black jumping spiders conceal them- 
selves between the fruits on the tree and are then able to catch the 
flies when they alight near them. Doubtless they destroy many in 
this way. On a few occasions ants have been observed attacking 
the maggots in a fruit which had fallen to the ground. They enter 
through the exit hole of the first maggot to escape and can then 
destroy the remaining larvae in the fruit. They represent a neg- 
ligible factor, however, in the control of the pest. 

Six hundred pupse dug from the soil under the trees and bred out 
in jars failed to produce a single parasite. The insect is well pro- 
tected from the attack of parasites through nearly the entire period 
of its life. 

CONTROL MEASURES. 

The most effective way of preventing injury from this pest is by 
bagging the trees or fruits. Either cheesecloth or mosquito netting 
can be tied over the trees or around the individual fruits, and the 
flies will not try to sting the fruits through it. However, this plan 
is hardly practicable on a large scale, since it requires considerable 
work arid expense and, in many cases, changing the bags as the fruits 
grow larger. 

The adults are readily killed by feeding them a poisoned sirup, 
the best results being obtained by using sodium arsenite or potassium 
arsenate dissolved in brown sugar sirup. When given this sirup 
the adult flies die very soon after feeding, and they eat it as readily 



10 BULLETIX 1081, U. S. DEPARTMENT OF AGRICULTURE, 

as the plain sirup. Very good killing results were also obtained 
by spraying this mixture with an atomizer on the under sides of the 
leaves of the trees in the large cages. Large numbers of flies were 
found dead on the ground within a couple of hours. These soluble 
poisons, however, burn the trees very severely and can not safely be 
used. Even at the rate of 1 pound to 50 gallons, which is as weak 
as can be effectively used, severe injury was noted. Insoluble ar- 
senic compounds such as Paris green, arsenate of lead, arsenate of 
calcium, and arsenite of zinc do not damage the trees but are not 
effective. When the arsenic is mixed in the sirup the flies do not 
get enough to kill them. 

The following plan if carried out thoroughly will very materially 
reduce the number of flies and make the growing of papayas prac- 
tical and profitable: (1) Selection of good seed and production of 
fruits of oblong shape and thick flesh which will offer more or less 
immunity to attack; (2) conscientious destruction of the infested 
fruits on the trees early in the season and before the maggots escape 
into the ground; (3) destruction of all inferior plants and wild 
plants around the place which might serve to breed the pests. 

If a planting is sufficiently isolated from other papayas the flies 
may be killed out by destroying all the plants in the spring, about 
April or May, and resetting new plants. These young plants will 
begin to fruit in the summer or early fall, but there will be a period 
of about 60 days when no fruits are present, which is long enough 
to starve out the flies. Along with this program should go the 
destruction of all wild plants in the hammocks for a radius of at 
least 2 miles. One large planting under observation was kept free 
from infestation for the entire winter by this method and a good 
crop of fruit obtained. The previous winter and spring the plants 
were badly infested, but the pests were entirely starved out during 
the summer. In most locations, however, a grower would not be 
sufficiently isolated to practice this method successfully unless the 
cooperation of his neighbors could be enlisted. 



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